1. Field of the Invention
The present invention relates to an apparatus and a method for analysis of trace-level gases, and more particularly to an apparatus and method for trace-level gas analysis, which eliminates the memory effect and enables detection of trace-level gas contaminants with a high accuracy.
2. Related Art
In semiconductor manufacturing processes, cases in which trace-level gas contaminants cause manufacturing defects are on the increase. In order to stabilize the manufacturing process, dust/chemical filters are used to remove offending matter. However, because of accidents involving contamination of materials used within the manufacturing process and filter damage, there is a need to achieve automated, continuous measurement and monitoring of the trace-level gas contaminants within the air.
In measuring trace-level components in the past, methods such as the impinger method were used to concentrate a component of interest over a long period of time up to the lower detection limit of the analysis apparatus, with analysis and quantification then being performed using an analysis apparatus of the ion chromatograph type, for example. This approach, however, was accompanied by the problem of a long measurement interval and the inability to identify the overall quantity of contaminant occurring, and the problem of not being able to accommodate suddenly occurring trace-level gas contaminants of a high concentration.
An additional problem arose because of the memory effect (influence of a previous measurement) when the component of interest remains in the sampling system or concentration system after high-concentration measurement using automatic ammonia analysis with the diffusion scrubber method, thereby preventing high-accuracy measurement.
In the multipoint gas analysis apparatus provided with a cleaning function as shown in FIG. 9 of the accompanying drawings, a cleaning fluid 43 is used so as to clean the switcher 41 and a diffusion scrubber 45. In this method, each time the measurement point of the sampling tube 40 is switched, because the switcher 41 and the diffusion scrubber 45 are cleaned before the measurement, the method is effective in eliminating influence of the point A on the measurement at point B. If point A only is observed, however, in a case in which the concentration at this point A changes suddenly (for example, when a sudden change from a high concentration to a low concentration occurs), even though the switch 41 and the diffusion scrubber 45 having been cleaned immediately previously are in a clean condition, because of a gas component that remains attached to the inner wall of the sampling tube 40, it is not possible to achieve a high measurement accuracy.
Accordingly, it is an object of the present invention, in order to improve on the above-noted drawbacks of the prior art, to provide a trace-level gas analysis apparatus and method which eliminate the memory effect and enable detection of a trace-level gas contaminants with a high accuracy.
To achieve the above-noted objects, the present invention adopts the following basic technical constitution.
Specifically, a first aspect of the present invention is a trace-level gas analysis apparatus comprising: a diffusion scrubber for capturing an atmospheric gas component; a sampling tube for guiding the captured atmospheric gas component to the diffusion scrubber; and cleaning means for cleaning the diffusion scrubber and the sampling tube using a cleaning fluid.
In the second aspect of the present invention, the apparatus further comprising: a pump for introducing the cleaning fluid into the diffusion scrubber and sampling tube and discharging the cleaning fluid from the diffusion scrubber and the sampling tube; and a valve for guiding the cleaning fluid to the diffusion scrubber and the sampling-tube and draining out spent cleaning fluid therefrom.
In the third aspect of the present invention, the apparatus further comprising: a plurality of sampling tubes for capturing atmospheric gas components at a plurality of locations; and a switching valve disposed between the plurality of sampling tubes and the diffusion scrubber so as to select any one of the sampling tubes.
In the fourth aspect of the present invention, the cleaning fluid is ultra-pure water.
The first aspect of the method for analyzing trace-level gas components of the air, in which the air is guided to a diffusion scrubber via a sampling tube, an absorption fluid is introduced into the diffusion scrubber, the absorption fluid being concentrated by a concentration column of an ion chromatograph, thereby separating and analyzing the atmospheric trace-level gas components, the method comprising steps of: a first step of cleaning the diffusion scrubber and the sampling tube; a second step of reclaiming the cleaning fluid used in the first step; a third step of causing an absorption fluid to circulate within the diffusion scrubber so as to stabilize capturing in the diffusion scrubber; a fourth step of causing an absorption fluid to circulate between the diffusion scrubber and the concentration column, so as to concentrate the trace-level gas components in the concentration column; and a fifth step of analyzing components concentrated in the concentration column using an ion chromatograph.
The second aspect of the method of the present invention is that in the fifth step, the first step and second step are performed simultaneously.
In the present invention, as shown in FIG. 1 of the accompanying drawings, a sampling tube 10 installed at a remote measurement point is connected to a diffusion scrubber 11, and the atmosphere sucked in thereat is supplied to the diffusion scrubber 11. During capturing operation, a trace-level gas component in the atmosphere is captured by setting a passage switching valve 24 to the side of an air pump 14, so that air is sucked into the diffusion scrubber 11, the absorption fluid 16 being caused to absorb the gas component. The capture of the trace-level gas by the diffusion scrubber 11, which is known by such disclosures as the Japanese laid-open patent publication (KOKAI) No.8-54380, is done by passing an ammonia component and a mono-ethanol amine component also with other trace-level gas components through a porous fluoride-based film within the diffusion scrubber 11, so that they are absorbed by the absorption fluid 16 passing through the porous fluoride-based film. The gas components absorbed by the absorption fluid 16 are subjected to concentration conversion by individual gas component, using concentration column 32, a separation column 33, a suppressor 34, and an electrical conductivity meter 35 of an ion chromatograph 3. During the process of concentration, that is, after the completion of sampling, the passage switching valve 24 is set to the cleaning fluid 22 side, and ultra-pure water 22 used as the cleaning fluid is sent by a cleaning pump 21 to the inside of the diffusion scrubber 11 and the inside of the sampling tube 10. Then, the passage switching valve 23 is set to the drain side, so as to cause the cleaning pump 21 to rotate in reverse, thereby reclaiming the cleaning fluid that had filled the inside of the diffusion scrubber 11 and the inside of the sampling tube 10, this completing the cleaning.
The operation of the present invention comprises a repetition of a cycle made up of a cleaning operation, in which cleaning fluid is supplied to the diffusion scrubber 11 and the sampling tube 10, a preparatory operation, in which the air and an absorption fluid 16 are caused to flow in the diffusion scrubber 11 and a porous fluoride-based film is placed in a state of equilibrium, a sampling operation, in which a gas component absorbed in the diffusion scrubber 11 is supplied to the concentration column 32 and the trace-level gas component is concentrated in the concentration column 32 of the ion chromatograph, and a separation and analysis operation, in which components concentrated in the concentration column 32 in the sampling operation are separated into individual components by the ion chromatograph.
In the present invention, by providing the cleaning section 2, which uses ultra-pure water to clean the sampling tube 10 and the diffusion scrubber 11 of the capturing section 1, it is possible to reduce the memory effect from the previous measurement, thereby enabling highly accurate monitoring of concentration. Immediately after sampling of a high-concentration gas in particular, there was the problem in the past of a high-concentration gas component becoming attached to the sampling tube 10 and diffusion scrubber 11 of the capturing section 1, thereby resulting in values higher than the actual concentration value being detected. With the present invention, however, because of the ability to eliminate the memory effect existing in the past, it is possible to perform measurements with high accuracy.